4 Reasons to Consider Normalizing Steel

October 24, 2013

Normalizing is a thermal process where  steel is heated about 100-150 degrees F  above the critical range followed by cooling in still air to below that range.

Not a fan of expensive thermal treatments without a good reason...

Not a fan of expensive thermal treatments without a good reason…

On some steels, this normalize process is followed by a temper or stress relief anneal below the Ac1 to remove residual stresses resulting from the air cooling and to reduce hardness.

 Normalizing Steel gives you

  • Reduced hardness and removes residual stress
  • Improves machinability
  • Develops desired mechanical properties (especially in larger sections)
  • Improved austenitizing  for subsequent quench and tempering

Adding costly thermal treatments to a production process is seldom a good idea. But

  • if you need high side mechanical properties as a result of the quench and tempering operation you have planned,
  • if the heat is lean on chemistry,
  • if the part to be quenched has a large cross section or wall thickness,
  • if you know from experience that you have difficulty getting to high side with your quench,

Normalizing can help.

For the end quench position of the bar that corresponds to 90% martensite,  a non-normalized  4140,  austenitized at 1550 degrees can have anywhere from a 7 to 14 point Rockwell C hardness deficiency compared to the same steel that had been normalized.  Using an austenitizing temperature of 1650 (200 degrees F above the Ac3 temperature) the non-normalized 4140 could still exhibit as much as a 10 Rc hardness deficiency compared to normalized stock for the same time at temperature. For 4340 steel, the hardness deficit can  range from 10 to 18 points of Rockwell C hardness deficiency for the same austenitizing time.

Normalizing was a necessary step in the days of highly variable microstructures resulting from Ingot steels and analog controlled processes. Today’s modern computer controlled steel making processes provide more consistent products and structures, making normalizing a less utilized process. But normalizing remains a way to coax better properties or performance out of some steels.

We’re not a big fan of adding “fire for fire’s sake,” but if you suspect you may have difficulty in developing the full hardenability out of your steel, Normalizing may just give you the edge you need to assure you develop the as quenched hardness you need.

Photo credit: Above The Law Blog


5 Ideas To Reduce Stress in Precision Machined Parts

October 6, 2009

Stress and stress risers are words that we often hear in our shops. Usually when bad things have happened to our work. Here are 5 ideas to reduce stress in precision machined products and a brief tutorial on what it is when the engineers say “stress.”

Looking at this, someone is going to say "stress riser."

Looking at this, someone is going to say "stress riser."

 

Stress (when used by designers, engineers, and metallurgists) refers to the measurement of load on a part  or test specimen related to the area under that load. Stress can be considered to be have three modes, axial (in line) , bending (you know what that means), or torsional (twisting or torque).  The following graphic illustrates some stress states our parts may encounter.

Diagram of Simple Stress States.

Diagram of Simple Stress States.

 

Residual stress can be considered to be a kind of “internal pressure” in the material which may act in the same direction as the stress applied to the part. Because of this, it can actually reduce the load carrying ability of the part. This is what usually results in failures. Characteristics of the part may also contribute to the concentration of these internal stresses, leading to premature failure of the parts once in service and subjected to load.

Here are 5 ideas to reduce stress in precision machined parts.

  1. Assure a smooth surface.
  2. Use a larger not smaller diameter for threading.
  3. Always maximize the fillet or radius between section or diameter changes.
  4. Provide both pads and relief areas on parts where applicable.
  5. Be alert to the fact that some materials are particularly notch sensitive, especially in the transverse direction.
Some things never change...

Some things never change...

Assure a smooth surface. The creation of a smooth surface prevents the concentration of internal stresses at sharp changes in surface. Parts with smooth surface finish are much less likely to fail than parts where deep grooves, tool marks or pits can allow stresses to build up.

Use a larger not smaller diameter for threading.  This is both related to the strength of the additional material as well as to the geometry and radii between change  of dimensions. The more generous radius possible with the larger diameter for threading can improve the endurance limit of the part substantially. In heat treated 4340, the increase in radius from 0.015″ to 0.090″ increase the endurance limit from 34,000 to 65,000 psi.

Always maximize the fillet or radius between section or diameter changes.  Any design which allows stress to concentrate locally will promote fatigue failure. Generous radii and fillets are inexpensive insurance against premature failure.

Make sure that the designer has provided both pad and releif areas on  parts joining perpendicularly.  Instead of having a single point or locus for the change in forces to be distributed through the part, pads and relief areas diffuse the stresses that would otherwise be concentrated,  improving the performance of the part.

Be alert to the fact that some materials are particularly notch sensitive, especially in the transverse direction. Many of the materials that we prefer to machine are resulfurized, and in these steels, the manganese sulfides can in fact lower the steel’s transverse mechanical properties. Also, cold drawing and or forging  prior to machining can influence grain flow which can enhance the ability of the material to carry the load. The material the designer selected could be a large reason for the material’s ability to handle stress, or not.

There you have it. Stress = Load. Don’t give it places to concentrate on your precision machined parts.

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